Using Classic Outlook on Windows for personal email, calendar, and contact management
No, not a windows update.
It's actually an ADOBE update that causes the issues with the Outlook delay rule. The ADOBE Reader Add-ins must be removed for the delay rule to work.
We had an Adobe update (which I suspected all along); added the add-ins back to Outlook and the rule stopped working.
Removing the Adobe Add-ins fixed the rule.
File - Options- Add -ins - COM Add-ins/GO - and uncheck the Adobe Add-ins.
Rule started working correctly again.